Electric explosion metal spraying for substrate

ABSTRACT

This invention relates to an improved process for coating a metal substrate with metallic particles by exploding a consumable metal wire in atmosphere or an inert gas of normal pressure by a heavy current.

g United States Patent 1151 3,639,150

Suhara el al. 1 Feb. 1, 1972 [5 1 ELECTRIC EXPLOSION METAL References Cd SPRAYING 11 OR SUBSTRATE OTHER PUBLICATIONS inventors! Tflflhil'oSuhal'il, 9-12 Komuin Jutaku, Chace A Survey of Exploding Wire Progress"from Explod Miyuki-rnachi, Kashii; Shigehisa Fukuda, ing Wires Vol. 3,Ed. by Chace et al., Plenum Press, New York 1479-3 Hakozaki, Hakata,both of Japan P1 [22] Filed: July 1968 7 Primary Examiner-Alfred L.Leavitt 21 LN 747 675 Assistant Examiner-J. H. Nev/some 1 App 0AttorneyLinton & Linton 1571 ABSTRACT This invention relates to animproved process for coating a [51] Int. Cl ..B44d l/02 metal Substratewith metanic particles by wading a Com sumable metal wire in atmosphereor an inert gas of normal [58] Field of Search ..117/105, 93.1, 93,105.2, 105.1 pressure by a heavy current.

2 Claims, 15 Drawing Figures mm FEB 11972 3s39;150

sum 1 or 2 FIG. 1.,

IN TmH/f juumzn MM BY HIGEHIJF FuKupA ELECTRIC EXPLOSION METAL SPRAYINGFOR SUBSTRATE The present invention relates to an improved method ofcoating of metals by applying on a substrate of metals, metalliccompounds and metal alloys, comprising the steps of exploding fineparticles of a consumable wire usable as spraying material in atmosphereor in an inert gas of normal atmospheric pressure by a heavy electricdischarge, causing impingement of the atomized particles with a veryhigh speed against the substrate surface to be coated to provide adense, smooth and highly adherent coating layer.

Wire explosion phenomena are well known, but it has been normally causedby an electric discharge of a heavy current through a metallic wire ofrelatively small diameter, say smaller than 0.1 mm., and the explosionvaporizes the wire into a high-pressure gaseous state. W. M. Connreported an explosion coating in the book, Exploding Wire, Volume 2,Plenum Press, New York (1962) or (Bulletin of the American PhysicalSociety" Volume ll, No. 2, Feb. 1966) wherein a silver wire of 0. l 6mm. in diameter was exploded by an electric discharge under a lowpressure (about I mm. Hg) of the air or inert gases. Upon following theprocedure described in the above-mentioned references the followingdifficulties have been found:

a. The electric discharge under the low pressure of air or inert gasused, ionized the gases around the wire through which a quantity ofelectric current passed, decreasing the explosive force and in turn theadhering strength of the coated particles.

b. Optimum operating conditions could not be established because ofuncertainty as to the quantity of electric current passing through thewire itself.

. The thickness of a coating layer adhered on a substrate by exploding asmall wire of the order of 0.l mm. in diameter was approximately 0.05micron for a single explosion which would be an impractical value.

One of the important objects of the present invention is to provide anovel coating process by using the optimum discharging conditions i.e.,using appropriate value of electric current, voltage and frequency in adischarging circuit and using a consumable wire having dimensions offrom 0.5 to 2.0 mm., superheating the wire instantaneously thus forminga mixture of high-pressure gas and the melt of the wire, and scatteringthe atomized particles at a high speed of 1-10 Machs. A coating by thepresent process here described, prevents the oxidation of the metalparticles because the highpressure gas produced will expel the ambientair or gas surrounding the wire.

Another object of the present invention is to provide a novel coatingprocess as described above which produces a uniform coating layer,unidirectional scattering and high yield of product applied.

The distinctive character of the present process is to control thedirection and speed of the atomized particles by utilizing the energy ofthe shock wave generated by the explosion of the consumable wire in theambient atmosphere or gas.

One of the advantages of the present invention is that the process issuitable to apply the coating on the undersurface of hollow cylindricalor spherical objects.

These and other objects and advantages of the present invention will beeasily appreciated by those skilled in the art from the followingdetailed explanation taken in connection with the accompanying drawingswherein:

FIG. 1 is a basic circuit diagram of electric exploding equipment usedin the practice of the present invention;

FIG. 2 shows an arrangement of a metallic pipe section, the innersurface of which is to be coated with a refractory metal such astungsten;

FIG. 3 is a recorded curve obtained by a surface-roughnessmeasuringinstrument which shows the superficial roughness of a coated surface ofthe sample coating shown in FIG. 4A having the composition by weight WC93.50 Co. 6.5;

FIG. 4A is a microphotograph of a section of a copper pipe coated withtungsten by this method;

FIG. 4B is a similar photograph to FIG. 4C showing a section of a copperpipe coated with molybdenum by this method;

FIG. 4C is a microphotograph of a section of a mild steel samplefour-ply coated with WC-Co alloy by this method;

FIG. 4D is a microphotograph of a section of a graphite sample four-plycoated with tungsten by this method;

FIG. 4B is a photograph of stainless-steel-coated iron (X I00);

FIG. 4F is a photograph of titanium-coated brass (X 450);

FIGS. 5A and 5B show the interaction between the atomized particles andthe shock wave reflected from a reflector by this method, FIG. 5A beingan axial view and FIG. 58 a side view;

FIG. 6A and 6B are microphotographs of the atomized particles and theshock wave produced by a. wire explosion; and

FIGS. 7A and 7B are photographs of mild steel surfaces coated by a WC-Cowire explosion respectively, without and with acrylic resin reflectors.

Referring now to the drawings, FIG. 1 diagrammatically illustrates agenerating circuit for an impulsive discharge current and a consumablewire to be exploded in accordance with the present invention, wherein awire 1 is mounted between a pair of electrodes 2,2 in the atmosphere orin an inert gas, and a heavy electric current is discharged through adischarging gap switch 6 and the wire 1 from a capacitor 3 of a largecapacity which is charged from a high-voltage DC generator 5 through acharging resistor 4. Because of the fact that an electrospark-formingcircuit is broadly known, a more detailed explanation of the circuitwill be omitted. The process illustrated by the circuit shown in FIG. 1is suitable to coat metals on the inner surface of cylindrical andspherical objects.

A feature of the present invention involves the utilization of the shockwave generated by the explosion of a consumable wire in the atmosphereor gas of normal pressure.

An advantage obtained by adoption of an inert gas has been proved byexperiments using a wire of aluminum, titanium and molybdenum, andnitrogen atmosphere which prevent the coated surface from adhesion ofoxidized particles on the substrate surface.

An explosion of a wire having its diameter less than 0.5 mm. produces athin coating layer of a lesser bonding strength because of a smallexplosion energy.

The shock wave propagated in the ambient gas by the explosion isreflected by reflectors which control .the flying direction and speed ofatomized particles of the wire itself resulting in a uniform coatinglayer of a high density and good adhering strength and a good yield ofapplied coated material by acceleration and concentration of theatomized particles.

FIG. 4E shows mild steel coated by an explosion of stainless steel wire(2 mm. diameter and 60 mm. long) using a current of 20 kv. and acapacity of 60 microfarads.

FIG. 4F shows brass coated by an explosion of titanium wire 1.6 mm.diameter and 60 mm. long, using a current of IO kv. and a capacity ofmicrofarads.

The principle of operation of this method is shown in FIGS. SA and 5B.

The shock wave produced by wire 11 is reflected by reflectors 12, 12positioned at an angle of 45 apart, into reflected waves 14, 14'traveling toward a substrate 13 to be coated, which converges theatomized particles 15 and I6 in the directions shown in dashed lines inFIG. 5. Thus, the wave intensity and its direction are controlled by thereflectors which in turn control the movement of the atomized particles.This practice is suitable for coating metals onto a plane surface. Asuitable distance between the wire and the substrate is 20 to 60 timesthe radius of the wire, preferably substantially 40 times.

The process of the present invention is conducted generally in thenormal atmosphere which is rather difficult to be ionized, such as atnormal air atmospheric pressure at sea level, resulting in prevention ofleakage of current through the ambient atmosphere. Accordingly, theelectric current concentrates in the wire I enabling the explosion of awire of a larger diameter, up to approximately 2 mm.

The wire is exploded after the selection of the dimension and wirematerial together with the optimum conditions of discharging, e.g., thedischarging voltage, the value of capacitor capacity and the resonantfrequency of the circuit to reduce the loss of electric energy.

The following table shows the results of the explosion spraying usingvarious kinds of substrate.

generates a superheating of the wire, producing instantaneously amixture of a high-pressure (of the order of 1,000 atmospheres) gas (lessthan 30 percent of the mass of wire), and melt. The gas ejects the meltat a speed of l-lO Machs in the radial directions.

As mentioned above, in this process, a wire of diameter 0.5-2.0 mm. isused in the explosion which produces a coating layer having a thicknessof more than 5 microns per one explosion.

Subtrate Al W Sprayed wire:

Brass (7:3) .1 Stainless steel (18:8). E WC-Co E Stainless Brass steelN1 M (7.3) (18.8

F E F E E E E E E E E E G E E F E E E F E E E F E E E E E E E F E ERemarks:

E =Excellent-Bond strength more than 100 kg./sq. em. G GoodBond strengthmore than 50 kg./sq. em.

F= Fair-Bond strength more than kg./sq. em

The optimum conditions of the present invention, for instance inexploding a tungsten wire, may be expressed by the following formulas:

These formulas have been proven to be correct by applicant, thenumerical constants have been determined empirically.

Suitable materials for the substrate to be coated include brass andstainless steel as shown in the above table.

A consumable wire of a diameter from 0.5 to 2 mm. is appropriate toperform the process. With a wire of a larger diameter, when ahigh-frequency current or an impulsive DC current pass through theconductor, the well-known skin effect" is produced resulting in thedisadvantage that uneven melting and evaporation of the wire result dueto the possible uneven quality of wire material. This statement appliesto most metal wires.

This disadvantage of unevenness would produce larger particles having adiameter of more than 100 microns which deteriorate the smoothness ofthe applied coated surface and adhering strength.

With a consumable wire of diameter less than 0.5 mm., a great portion ofthe wire is exploded into an explosion gas, and the diameter of theatomized particles is less than l micron, and hence the smaller energyof impingement against the substrate surface will produce a pooradhesion.

In the process of the present invention, using a consumable wire ofdiameter 0.5 to 2.0 mm, a discharge through the wire The following areexamples illustrating the practice of the present invention.

EXAMPLE 1 At the axial position of the section of hollow copper pipe A(FIG. 2) having a length of 30 mm. and inside diameter of 30 mm.,machined, polished and washed with trichloroethylene, a tungsten wire Bhaving a length of 50 mm. and diameter of 1 mm. was mounted between apair of electrodes C, C as shown in FIG. 2. The copper pipe A wassupported on an insulating body D.

When the charged energy of 4 kilojoules in condenser 3 is dischargedduring about 10 microseconds. the tungsten wire 1 partly vaporizes bythe sudden superheating and the remainder of the wire melts and isexploded into fine particles by the explosion gas and is ejected at ahigh speed of l-l0 Machs against the inner surface of the pipe producinga tungsten coating thereon.

The explosion is made generally under normal atmospheric pressure, butbecause of the fact that the explosion gas precedes the atomizedparticles pushing away the air by a distance of approximately l0 timesthe pipe radius, the particles are not oxidized.

The surface roughness of the applied coated layer by the presentinvention, is within the variation of approximately 5 microns as shownin the record curve of FIG. 3 recorded by a roughness tester and thecoated surface is very smooth. The density of the layer is measured as18 g./cc. which corresponds to 93 percent of the theoretical density andits structure is very dense as shown in the microphotograph of FIG. 4A.

The above-mentioned process was repeated with a molybdenum wire having alength of 50 mm. and diameter of 1.4 mm., and the sectionalmicrophotograph of the coated layer obtained is shown in FIG. 4B. Theresult of the adhering test on the coated layer shows an excellent valueof bond strength of more than 226 kg./sq. cm. as compared with anaverage value of 173 kgJsq. cm., as compared with an average value of713 kg./sq. cm. which has been obtained by a prior spraying gun processof the prior art.

A voltage more than 50 kv. is apt to ionize the air and produce a coronadischarge which makes the charging equipment hard to be handled.Furthermore, a high voltage will cause an electrical resistance riseduring an explosion of wire and hence a direct discharge often occursbetween an electrode and the substrate. No critical value exists incapacitor capacitance.

EXAMPLE 2 A schematic diagram of the operation of this example is shownin FIGS. 5A and 5B. As a consumable wire 11, a tungsten wire having alength of 30 mm. and diameter of 1 mm. was connected to electrodes atboth its ends, and acrylic resin plates as reflectors 12, 12' were setapart from the wire by 5 mm., said plates being mounted in a divergingposition at an angle of 45 to one another and on an angle 0 to mountingbase 17. A metallic plate 13 to be coated was positioned opposite thediverged side of the reflectors 20 mm. apart from the wire. When thereis an explosion by an electric discharge under conditions of chargingvoltage 20 kv., applied to the capacitor of 20 microfarads under acharging energy of 4 kilojoules passed through the wire, the resultingcoated layer on plate 13 had a thickness of 1 1 microns. With thesereflectors 12, the thickness of the obtained coated layer on plate 13was only 7 microns.

In FIGS. 2A and SB, 10, 10 are electrodes; 11 is the wire; 12, 12' arereflectors; 13 is a substrate; 14, 14' are reflected shock waves; 15, 16are deflected spraying particles; 17 is a mounting base for thesubstrate; 18 is the shock wave.

The wire llll is mounted in a position parallel to and apart from thesubstrate by a suitable distance e.g., 40 times of the wire radius, andthe reflectors are arranged at an angle of 60-l 20bL relative to saidmounting base 17.

A part of the cylindrical shock wave generated by the explosion of thewire is reflected by the reflectors 12, 12 forming the reflected waves14, 14', which are directed towards the substrate with the inwardlyinclined reflectors. A group of particles 15, 16 flying in the radialdirection are deflected in a direction shown by an arrow by the waves14, 14 and collected on the substrate surface. In this case thevelocities of the shock wave and particles are approximately l,500-2,000m./sec. and 30-1 ,000 m./sec. respectively. Thus, the direction ofparticle spraying may be controlled by the reflectors.

Accordingly, the thickness of coating per one explosion may beincreased.

Adjusting the inclination of the reflectors 12, 12' from 60 to 120relative to the mounting base 17 will control the flying direction ofthe particle toward the substrate effecting its concentration or uniformdistribution of the particle.

Application of low-velocity particle or oxidized fume on the substrateis in every respect disadvantageous which will cause surface stain andobstruction to the succeeding spraying, but since the kinetic energy ofthese particles is relatively small, reflectors 12, 12' arranged at anangle 0 of 90-l20 relative to said mounting base 17 generate thereflected waves 14, 14 which expel these particles outside the substrate13. Therefore, a clean surface may be obtained as shown in FIG. 7B,whereas without the reflectors the surface has been stained with theseparticles as shown in FIG. 7A.

The relationships between the shock wave generated by the explosion andthe atomized particles are shown in FIG. 6A and FIG. 6B obtained byusing a high-speed (200,000

frames/sec.) camera. The microphotograph in FIG. 6A was taken at aninstant I/40,000 second after the explosion wherein the central whiteportion is a mixture of explosion gas I and atomized particles and theouter circle shows a wave front of the shock wave just reaching theplate to be coated. By this moment both the explosion gas and theatomized particles are expanding uniformly, but as shown in themicrophotograph of FIG. 6B taken l/200,000 second after FIG. 6A, areflected shock wave from the plate 12 is obviously striking against thefront of explosion gas and atomized particles affecting theirpropagations. In this event, a spherical collision is caused between thereflected shock wave and the propagating gas, and a resulting plane wavereaches the metallic plate forming a uniform-coated layer.

FIG. 7A and FIG. 7B are photographs of mild steel surfaces coated by aWC-Co wire (diameter of 1 mm. and length of mm.) explosion in a nitrogenatmosphere under same conditions (1 l kv. and 80;.tf.) respectively,without and with acrylic resin reflectors l2, 12 disposed at an angle ofto mounting base 17, the former (A) apparently shows black status byoxidized particles of lower velocity whereas (B) demonstrates a clearsurface coated by nonoxidized particles.

The process described has proved to give results similar to the case inwhich the explosion was conducted under normal atmospheric pressurevalue of an inert gas such as nitrogen because of the fact that theexplosion gas effectively expels the air or gas surrounding the wire.

We claim:

1. A method of spray coating metallic materials on a substratecomprising the steps of positioning a substrate to be coated at adistance from a wire of said metallic material mounted between a pair ofelectrodes under atmospheric pressure, said distance of said substratefrom said wire being substantially 40 times the radius of said wirehaving a diameter of 0.5-2.0 mm. and a length of 20-300 mm., dischargingan impulsive electric current in accordance with the formulas:

K I is a material constant (:20percent) K is a constant (:30 percent) C=capacitor capacitance (farad) V=charging voltage (volts) f =resonantfrequency of the circuit (c.1lsec.)

l opt: Optimum length of a wire (mmr), through the wire,

explosively spraying 50-60 percent of the wirefused particles havingI-10 microns diameter and the remaining 40-50 percent of the wire beingvaporized and providing a coating having 5-15 microns thickness at atime on surfaces of the substrate.

2. The method of spray coating as claimed in claim 1 including thefurther step of reflecting said exploded material from inclinedreflectors situated on both sides of said wire and said substrate.

2. The method of spray coating as claimed in claim 1 including thefurther step of reflecting said exploded material from inclinedreflectors situated on both sides of said wire and said substrate.